With the continuous development of optical fiber transmission technology, the Erbium-Doped Fiber Amplifier (EDFA) and Wavelength Division Multiplexing (WDM) technology have been used since the mid-1990's. In a WDM system, since the insertion loss brought about by the multiplexer and de-multiplexer used therein is relatively high, amplification and compensation are generally performed by EDFA. However, when the optical power is amplified, the nonlinear effects in the optical fiber are increased greatly. The nonlinear effects comprise four-wave mixing, self-phase modulation, cross-phase modulation, etc, which would restrict the capacity and distance of the optical transmission. Hence, in the transmission systems of large capacity and high speed, a higher requirement on the performance of the transmission fiber is put forward. The nonlinear effects in the optical fiber can be reduced through improving the performance of the optical fiber.
When a system with a high power density is used, the nonlinear coefficient is used to evaluate the effects on the performance of the system brought about by the nonlinear effects. The nonlinear coefficient is defined as n2/Aeff, wherein n2 is the nonlinear refractive index of the transmission fiber, and Aeff is the effective area of the transmission fiber. It can be seen that, the nonlinear effects in the optical fiber can be reduced through improving the effective area thereof.
The effective area of the optical fiber is associated with the mode field distribution thereof. According to the G.650.2 standard formulated by the International Telecommunication Union Telecommunication Standardization Sector (ITU-T), the empirical formula of the relation between the effective area of the optical fiber and the mode field distribution thereof is expressed as:Aeff=kπw2  (1),wherein k is a correction factor, which is different for the optical fiber with different refractive index profiles. In the above formula (1),
      w    =          MFD      2        ,wherein MFD is the mode field diameter of the optical fiber. It can be easily seen that, if MFD is increased when the optical fiber is designed, the Aeff thereof can be enlarged accordingly. It is known by a person skilled in the art that, the MFD of the optical fiber can be increased through regulating the refractive index of the core layer and the core diameter thereof. However, the increasing of the MFD of the optical fiber would adversely affect other performances of the optical fiber, such as the cutoff wavelength and the bending performance.
Theoretically, the bending performance of the optical fiber depends directly as the value of MAC thereof. That is to say, the larger the value of MAC is, the poorer the bending performance will be. The value of MAC is the ratio of the mode field diameter of the transmission fiber to the cutoff wavelength thereof. In order to guarantee a single-mode transmission in the optical fiber with a large effective area within the application wavelength window, the cutoff wavelength cannot be too high. For example, according to the G.654 standard formulated by ITU-T, the cable cutoff wavelength is suggested not to surpass 1530 nm. Hence, since the cutoff wavelength is limited in a certain numerical scope, the increasing of MFD would increase the value of MAC inevitably. In this case, the bending performance of the optical fiber is adversely affected. Therefore, as to the designing of optical fiber with a large effective area, the key lies in the balance among respective parameters thereof, so that a reasonable compromise among the performances of the optical fiber can be obtained.
An optical fiber with an effective area larger than 150 μm2 is proposed by Chinese patent No. 102313924 A. In the optical fiber according to the patent, a depressed cladding layer is added outside the cladding layer to inhibit the deterioration of the bending performance thereof. The improvement of the bending performance of the optical fiber is dependent on the volume of the depressed cladding layer. The larger the effective area of the optical fiber is, the larger the volume of the depressed cladding layer will be. The disadvantage of the above method lies in that the increasing of the volume of the depressed cladding layer would increase the cutoff wavelength thereof. Therefore, in order to obtain a larger effective area, the restriction on the cutoff wavelength is relaxed in the above patent. In its embodiments, the cutoff wavelength of many samples surpasses 1530 nm, or even reaches 1800 nm and more. Evidently, the optical fiber cannot totally satisfy the wide application at a wavelength of 1550 nm.
The electric field density distribution of the light transmitting in the optical fiber can be changed through adding a depressed structure to the core layer of the optical fiber. In this case, the distribution curve will become flatter. This means that, with reasonable design, the optical fiber with a depressed structure at its core layer could have a larger effective area compared with the ordinary optical fiber with a step profile on the premise of their respective mode field diameters are the same with each other. It can be understood that, in the above formula (1), the optical fiber with a depressed structure at its core layer would have a larger value of k. Then, under the circumstances of the increasing of MFD is limited, the structure would facilitate the further improving of the effective area thereof. A non-zero dispersion-shifted optical fiber with a depressed core layer is proposed by “Non-zero dispersion-shifted optical fiber with ultra-large effective area and low dispersion slope for terabit communication system” (Optics Communication 236 (2004) P69-74). It is described in the paper that, the electric field strength distribution of the optical fiber with a depressed core layer is different from that of the traditional non-zero dispersion-shifted optical fiber with a step profile, and the difference is considered to be the main reason of the significant improving of the effective area of the former optical fiber.
An optical fiber with a large effective area is proposed by U.S. Pat. No. 6,904,218 B2. Said optical fiber comprises a core layer, a depressed layer, and a cladding layer. In one of its embodiments, an optical fiber having a parabola-shaped sectional profile of refractive index at its core layer is proposed. On this basis, a certain change is performed. That is, the parabola, i.e., the sectional profile of refractive index of the core layer, is made to deviate from the central axis of the core layer. The changed sectional profile has a certain effect on improving the effective area of the optical fiber. However, in the embodiments of said patent, the effective area of the optical fiber at a wavelength of 1550 nm is only 131.2 μm2.
Therefore, as to the designing of the optical fiber with large effective area, the challenge is to obtain a desirable compromise among its performances, such as the cutoff wavelength, the bending performance and the like, through a reasonable designing of the sectional profile of the optical fiber. In this manner, the effective area of the transmission fiber can be improved as much as possible, and the nonlinear effects thereof can be reduced, so that the optical fiber can be widely used in the transmission systems of large capacity and high speed.
Some of the terms in the present invention are defined and explained hereinafter. According to the different refractive indexes, the layer which is the nearest to the central axis of the core layer of the optical fiber is defined as the first sub core layer, and the outmost layer of the optical fiber, i.e., the pure silicon dioxide layer, is defined as the cladding layer of the optical fiber. From the first sub core layer to the cladding layer of the optical fiber, there are the first sub core layer, the second sub core layer, and so on, in sequence.
The relative refractive index difference Δni of each layer of the optical fiber is defined as:
            Δ      ⁢                          ⁢              n        i              =                                        n            i                    -                      n            c                                    n          c                    ×      100      ⁢      %        ,wherein ni is the refractive index of each layer of the optical fiber, and nc is the refractive index of the cladding layer, i.e., the refractive index of pure silicon dioxide.
The effective area Aeff of the optical fiber is defined as:
            A      eff        =          2      ⁢      π      ⁢                                    (                                          ∫                0                ∞                            ⁢                                                E                  2                                ⁢                r                ⁢                                                                  ⁢                                  ⅆ                  r                                                      )                    2                                      ∫            0            ∞                    ⁢                                    E              4                        ⁢            r            ⁢                                                  ⁢                          ⅆ              r                                            ,wherein E is the electric field relating to transmission, and r is the distance from the central axis of the optical fiber to the distribution points of the electric field.
It is defined in the 60793-1-44 standard formulated by the International Electrotechnical Commission (IEC) that, the cable cutoff wavelength is the wavelength when the optical signal does not transmit as a single-mode signal any more after transmitting 22 meters in the optical fiber. In test, one turn of the optical fiber with a bending radius of 14 cm and two turns of the optical fiber with a bending radius of 4 cm shall be made to obtain the data.